1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device having a circuit comprising a thin film transistor (hereinafter referred to as a TFT), for example, an electro-optical device represented by a liquid crystal display panel and an electronic device in which such an electro-optical device is mounted as parts.
Note that in this specification, a semiconductor device indicates a general device functioning by utilizing semiconductor characteristics, that is, an electro-optical device, a semiconductor device, and an electronic device each are a semiconductor device.
2. Description of the Related Art
In recent years, a technique of structuring a thin film transistor (TFT) using a semiconductor thin film (about several to several hundred nm in thickness) formed on a substrate having an insulating surface is drawing attention. The thin film transistor is widely applied to an electronic device such as an IC or an electro-optical device. In particular, the development for a switching element of an image display device is being advanced.
A material of a crystalline semiconductor film suitably used for the TFT is silicon. A silicon film having a crystalline structure (hereinafter referred to as a crystalline silicon film) is generally formed through the following process. That is, an amorphous silicon film is deposited on a substrate made of glass, quartz, or the like by a plasma CVD method or a low pressure CVD method and then crystallized by heating treatment or laser light irradiation (hereinafter referred to as laser processing in this specification) to obtain the crystalline silicon film.
However, in the case of the crystalline silicon film produced by the above conventional method, the crystal orientation planes are present randomly and an orientation ratio with respect to a specific crystal orientation is low. When the orientation ratio is low, it is almost impossible to keep continuity of lattices because of a grain boundary produced by collision of crystals with different orientations. Thus, it can be estimated that a large number of dangling bonds are produced. Dangling bonds which can be produced in the grain boundary become trapping centers of carriers (electron and hole) to reduce transporting characteristics. That is, since the carriers are scattered or trapped, even when a TFT is manufactured using such a crystalline semiconductor film, a TFT having high field effect mobility cannot be expected.
Also, a large number of distortions, defects and the like are present in a silicon film having a crystalline structure, that is, in a polysilicon film. They function as traps of carriers to deteriorate electrical characteristics. Thus, even in the channel forming region of the TFT, an existence configuration of a distortion, a volume of a lattice defect, and the like become large factors for causing a variation in characteristic.